1. Field of the Invention
The present invention relates to polymer recycling, and in particular, to a method for separating technical and biological polymers by means of liquid-liquid separation.
2. Background of the Invention
Polymer recycling involves separating polymers. This separation could mean: (1) the separation of polymers of one type according to molecular weight; or (2) the separation of different types of polymers (for example polystyrene and polyester) regardless of molecular weight.
Polymers, in this case technical polymers, are large-scale products in the chemical industry with production reaching approximately 6.4 million tons in 1990. By far the majority of the produced polymers are thermoplastics, in other words, polymers with finite molecular weight. Thermoplastics represent approximately 90% of the produced amount and can be subdivided into approximately 11 different chemical types such as, for example, polyvinyl chloride, polystyrene, polyolefin, polyester, copolymers and so forth. Recycling depends on whether the different types that form part of mixed waste, can be separated into pure substances since mixed polymers usually do not result in a high quality product.
The recycling can be performed in accordance with the following methods:
Thermal recycling: In other words, combustion of the mixed waste for the extraction of the combustion heat. This method converts the polymers into the substances CO2 and H2O, which are basically not suitable for further synthesis.
Recycling according to type: This type of recycling is usually reserved for the processor, since commercial polymers usually are mixed.
Mechanical separation: This method takes advantage of the different polymer densities or the different wetting characteristics of the polymers. It is not suitable for hollow or mechanically connected parts consisting of different polymers nor for polymers that contain additives (softeners, flame-retardants and others).
Type recognition mechanical separation: This method relies on fast analysis methods that recognize the polymer type and sort the recognized parts. This method is not suitable for mechanically connected parts consisting of different polymers.
A common disadvantage of all the above-mentioned separation methods is the fact that they cannot respond to the additives and auxiliary agents nor decomposed polymers that are often present (for example as a result of the use of UV radiation).
Thermal separation methods should also be considered for polymer separation. Due to the very low vapor pressure of polymers, distillation cannot be considered, leaving extractive and adsorptive methods as possibilities.
When examining the solubility of a polymer in a solvent it becomes apparent that it is very dependent on both the temperature and the molecular weight of the polymer. FIG. 1 illustrates this correlation.
In FIG. 1, the polymer concentration in a solvent (0 to 100%) is plotted against temperature. The marked two-phase boundaries (1) and (2) separate a region of total miscibility (4) from the regions (3) or (5) of separation into two liquid phases, one of which is rich in solvent and the other is rich in polymers. It is generally accepted that two separation areas exist, one at lower temperatures (3) and one at higher temperatures (5). An increase in polymer molecular weight leads to an expansion of the two-phased separation and, in extreme cases, leads to the formation of a single connected two-phase area (watch-glass diagram).
For technical separation tasks, separation at higher temperatures is particularly interesting, since at lower temperatures and with high polymer content ( greater than  20 wt. % polymer), the viscosity of the solution is very high, which leads to difficulties with material transfer and heat transfer and hence leads to higher costs (high-viscosity technique). The two-phase area (5) can be shifted to lower temperatures (see, for example, B. Bungert, G. Sadowski, W. Arlt, Fluid Phase Equilibria 139 (1997) 349-359) by compressing a gas, which is particularly useful for thermally unstable polymers. In this manner, the advantages of low viscosity can be combined with the advantage of lower temperature requirements. An additional advantage when using a compressed gas is that, as opposed to liquid precipitants (through the use of which a separation also could be achieved), it can be quasi-quantitatively removed from the system when the pressure is lowered, eliminating the need for additional processing steps such as, for example, distillation.
If two polymers in one common solvent are examined, a segregation into two liquid phases usually occurs as soon as the total polymer content reaches a value of 5-10 wt. % even when both the pure polymers are completely soluble in the solvent being examined (see, for example, S. Krause, J. Macromol. Sci.xe2x80x94Revs. Macromol. Chem., C7(2), (1972) 251-314). FIG. 2 illustrates these interrelations using a triangle diagram. The triangle diagrams of FIG. 2 and 3 are in accordance with J. Gmehling, B. Kolbe, Thermodynamics, 2nd edition, Weinheim 1992.
Referring to FIG. 2, while the corners of the triangle represent the pure substances polymer P1, polymer P2 and the solvent LM, the sides of the triangle represent the corresponding binary subsystems. All possible concentrations of the ternary system polymer P1/polymer P2/solvent LM are located within the triangle surface. Line 8 separates the area of complete miscibility (9) from the area (10) of separation into liquid phases. In area 10, two phases are formed that contain mainly one of the respective polymers as well as additional detectable amounts of the other polymer. A polymer separation with a high degree of purity ( greater than 99%) taking advantage of this segregation is theoretically possible when working with total polymer content of between 30-50 wt. %. In practice, however, it is not feasible since when doing this, two (or more) phases are obtained, each with a high polymer content. These phases are so viscous that a mechanical separation of the phases is not possible.
WO091/03515 to E. B. Nauman describes a method for the selective dissolving of a polymer mixture using a specifically chosen solvent. The disadvantages of this method are the extensive use of solvent, low selectivity and over sensitivity with regard to the molecular weight of the polymer. Additionally, the dissolving and separation of the polymers is only partially successful due to the high viscosity in the solution. Nauman himself points out that the method is not suitable for composite materials consisting of different polymers.
An additional thermal separation method is liquid chromatography that can be controlled in such a way that a separation of polymers is achieved regardless of molecular weight (see, for example, W. Arlt, A. Lawisch, German Patent Application, File No. 197 14063.7-41). The separation of one polymer type is achieved using distribution between a mobile and a stationary phase. Such a chromatography installation consists of the following parts: machine used to maintain the flow of the mobile phase; feeder device for the substances that are to be separated; column with the stationary phase; detector for the polymer types that are to be separated. The described installation configuration is employed for analytical purposes and under certain circumstances also for technical separation. In this connection, the column material, solvent and elution agent must be chosen in such a manner that the retention times for all types are finite, in other words, so that none of the types irreversibly adhere to the column (again, see, for example, W. Arlt, A. Lawisch, German Patent Application, File No. 197 14063.7-41). The limitations of this method are that it is necessary to work with very dilute solutions because of the modest simultaneous solubility of several of the polymers in the common solvent at simultaneous high viscosity, which affects costs.
The method according to the invention offers a significant improvement over prior methods in that it uses a two-stepped liquid phase separation in which at most one of the phases has higher viscosity. The fundamental principle of the method is described in the Detailed Description for two polymers for the sake of clarity, but is also applicable for more than two polymers.
A first embodiment of the present invention provides a method for the separation of technical or biological polymer mixtures as follows:
the polymers are technical or biological macro molecules with a number average molecular weight of greater than 1000 g/mol and less than 1000 kg/mol and are at least partially soluble in at least one suitable solvent;
the mixtures are not completely miscible in the melt;
the above-mentioned mixtures are dissolved in a solvent or solvent mixture in such a manner that the solution contains 1-50 wt. %, preferably 5-25 wt. % total polymer and such that at least two liquid phases are formed;
the solvent or solvent mixture has an atmospheric boiling point between 30xc2x0 C. and 250xc2x0 C.;
by decanting the liquid phases, homogenous fractions are formed in which a polymer or a group of chemically similar polymers (target polymer) is present in increased concentration;
the homogeneous fractions are subjected to pressures and temperatures, at which the pure target polymer with the solvent form two liquid phases so that the fractions separate into at least two phases of which the first is a liquid polymer rich phase that contains the target polymer with an even higher degree of purity and the second of which is a polymer poor liquid phase; and
the polymer rich phase is separated and the target polymer is extracted with an even greater degree of purity.
A second embodiment of the present invention provides a method for the separation of technical or biological polymer mixtures as follows:
the polymers are technical or biological macro molecules with a number average molecular weights of greater than 1000 g/mol and less than 1000 kg/mol and that they are at least partially soluble in at least one suitable solvent;
the mixtures in the melt are not completely miscible;
the above-mentioned mixtures are dissolved in a solvent or solvent mixture in such a manner that the solution contains 1-50 wt. %, preferably 5-25 wt. % total polymer and such that at least two liquid phases are formed;
the solvent or solvent mixture has an atmospheric boiling point between 30xc2x0 C. and 250xc2x0 C.;
by decanting the liquid phases, homogenous fractions are formed in which a polymer or a group of chemically similar polymers (target polymer) is present in increased concentration;
one or more fractions is mixed with a compressed gas or gas mixture to achieve the forming of two or more liquid phases so that the fractions are separated into at least two liquid phases, the first of which is a polymer-rich phase that contains the target polymer with an even higher degree of purity, and the second of which is a polymer-poor liquid phase; and
the gas is a substance or a substance mixture that is available at 1 bar absolute and at 25xc2x0 C. in a gaseous state.
A third embodiment of the present invention provides a method for the separation of technical or biological polymer mixtures as follows:
the polymers are technical or biological macro molecules with a number average molecular weight of greater than 1000 g/mol and less than 1000 kg/mol and that they are at least partially soluble in at least one suitable solvent;
the mixtures in the melt are not completely miscible;
the above-mentioned mixtures are dissolved in a mixture consisting of at least one solvent and one gas, in such a manner that the solution contains 1-50 wt. %, preferably 5-25 wt. % total polymer, and such that at least two liquid phases are formed;
the solvent or solvent mixture has an atmospheric boiling point between 30xc2x0 C. and 250xc2x0 C.;
by decanting the liquid phases, homogenous fractions are formed in which a polymer or a group of chemically similar polymers (target polymer) is present in increased concentration;
one or more fractions are mixed with another compressed gas or gas mixture;
the gas is a substance or a substance mixture that is available at 1 bar absolute and at 25xc2x0 C. in a gaseous state;
the fractions obtained in accordance with step (f), are subjected to pressures and temperatures at which the pure target polymer with the solvent form two liquid phases so that the fractions separate into at least two liquid phases, the first of which is a polymer-rich phase that contains the target polymer in an even higher degree of purity, and the second of which is a polymer-poor phase; and
the polymer-rich phase is separated and the target polymer is extracted with an even higher degree of purity.
A fourth embodiment of the present invention provides a method for the separation of technical or biological polymer mixtures as follows
the polymers are technical or biological macro molecules with a number average molecular weight of greater than 1000 g/mol and less than 1000 kg/mol and that they are soluble in at least one suitable solvent;
the mixtures in the melt are not completely miscible;
the above-mentioned mixtures are dissolved in a solution or solution mixture in such a manner that the homogeneous solution contains 1-50 wt. %, preferably 5-25 wt. % total polymer;
the solvent or solvent mixture has an atmospheric boiling point between 30xc2x0 C. and 250xc2x0 C.;
the solution is exposed to rising temperatures so that the polymers in the solution are subjected to the following subsequent steps:
ea) the temperature, above which the respective polymer with the pure solvent forms two liquid phases, is exceeded so that the solution separates into at least two liquid phases, the first of which is a liquid polymer-rich phase that contains the respective enriched polymer (target polymer) and the second of which is a polymer-poor phase;
eb) the polymer-rich phase is separated and the target polymer is extracted with a higher degree of purity; and
ec) the polymer-poor phase is subjected to steps (ea) to (eb) until all desired polymers are separated.
A fifth embodiment of the present invention provides a method for the separation of technical or biological polymer mixtures as follows:
the polymers are technical or biological macro molecules with molecular weight of greater than 1000 g/mol and less than 1000 kg/mol and that they are soluble in at least one suitable solvent;
the mixtures in the melt are not completely miscible;
the above-mentioned mixtures are dissolved in a solvent or solvent mixture in such a manner that the homogenous solution contains 1-50 wt. %, preferably 5-25 wt. % total polymer and such that at least two liquid phases are formed;
the solvent or solvent mixture has an atmospheric boiling point between 30xc2x0 C. and 250xc2x0 C.;
the solution is exposed to falling temperatures so that the polymers in the solution are subjected to the following subsequent steps:
ea) the temperature fall below the point at which the respective polymer with the pure solvent forms two liquid phases, so that the solution separates into at least two liquid phases, the first of which is a polymer-rich phase that contains the respective enriched polymer (target polymer) and the second of which is a polymer-poor phase;
eb) the polymer-rich phase is separated and the target polymer is extracted with a higher degree of purity; and
ec) the polymer-poor phase is subjected to steps (ea) to (eb) until all desired polymers are separated.
A sixth embodiment of the present invention provides a method for the separation of technical or biological polymer mixtures as follows the polymers are technical or biological macro molecules with a number average molecular weight of greater than 1000 g/mol and less than 1000 kg/mol and that they are soluble in at least one suitable solvent;
the mixtures in the melt are not completely miscible;
the above-mentioned mixtures are dissolved in a solvent or solvent mixture in such a manner that the homogenous solution contains 1-50 wt. %, preferably 5-25 wt. % total polymer;
the solvent or solvent mixture has an atmospheric boiling point between 30xc2x0 C. and 250xc2x0 C.;
the solution is exposed to a constant portion of a compressed gas or gas mixture (gas content) and rising temperature, or is mixed with increasing portions of a compressed gas or gas mixture (gas content) at a constant temperature, so that the polymers in the solution are subjected to the following subsequent steps:
ea) the temperature or the gas content, above which the respective polymer with the pure solvent and gas or gas mixture forms two liquid phases, is exceeded so that the solution separates into at least two phases, the first of which is a liquid polymer-rich phase that contains the respective enriched polymer (target polymer) and the second of which is a liquid polymer-poor phase;
eb) the polymer-rich phase separates and the target polymer is extracted with a higher degree of purity;
ec) the polymer-poor phase is exposed to the steps in accordance with characteristics (ea) to (eb) until all desired polymers are separated; and
the gas is a substance or a substance mixture that is available at 1 bar absolute and at 25xc2x0 C. in a gaseous state.
A seventh embodiment of the present invention provides that, in any of the methods of the first six embodiments, the polymers to be separated are co-polymers with various co-monomer content.
An eighth embodiment of the present invention provides that, in any of the methods of the first seven embodiments, the short chain content of the molecular weight distribution of the enriched polymer accumulates with an increased degree of purity in the solvent-rich phase and therefore is depleted in the polymer-rich phase.
A ninth embodiment of the present invention provides that, in any of the methods of embodiments one, two, or three, after the first decanting step, the homogeneous fraction which contains a single polymer or a group of chemically similar polymers, after drying of the initial solvent or without any other processing, is dissolved in another solvent or solvent mixture.
A tenth embodiment of the present invention provides that, in any of the methods of embodiments two, three, and six, the gas content and the temperature are chosen in such a manner that the time needed until 99 wt. % of the substances have settled in phases has been reduced to under 20 minutes.
An eleventh embodiment of the present invention provides that, in any of the methods of embodiments one through ten, the polymers are thermoplastics or their mixtures.
A twelfth embodiment of the present invention provides that, in any of the methods of embodiments one through ten, the polymers are polystyrene, polyvinyl chloride, polyolefin, or their mixtures.
A thirteenth embodiment of the present invention provides that, in any of the methods of embodiments one through twelve, the solvent is an aliphatic, aromatic or cyclic, saturated or unsaturated hydrocarbons, alcohols, carboxyl acids, amines, esters, ketones, aldehydes, ethers, water, tetrahydrofurane, dimethylformide, dimethylsulfoxide, n-methylpyrrolidone, n-methylcaprolactam, or a mixture thereof.
A fourteenth embodiment of the present invention provides that, in any of the methods of embodiments two, three, and six, the utilized gas is a saturated or unsaturated hydrocarbon, nitrogen, nitrous oxide, halogenated hydrocarbons, ammonia, inert gases, or a mixture thereof.
A fifteenth embodiment of the present invention provides that, in any of the methods of embodiments one through fourteen, the method is suitable for reprocessing used plastics.